Actual dimension of Omega in Graph Slam

I have a doubt about GraphSlam Algorithm introduce in this paper. I add a picture to express my doubts (from page 411, 9th in paper):

As per my knowledge $\Omega$ is the information matrix. So in this algorithm as per 2nd line it is mentioned that $\Omega = 0$. How can I set one zero to a matrix? It is a two dimensional array. So I need an array of values.

2nd doubt: As per line no 3 a $3x3$ matrix add to $\Omega$. How could it be possible because if we turn the page we can see a in line 7 a 6*6 matrix add to $\Omega$. What is the actual meaning of line number 3.

3rd doubt: What is the meaning of $x_0$? Is it a column vector of pose? meaning it represent $x,\ y,\ \theta$?

I want clarify my doubt using a picture.

What is the meaning of adding $\Omega$ at $x_0$?